I have assisted in an Autism class and I sometimes substitute in Autism classes, and a lot of the children have trouble with using the potty. Many of them are not potty trained even in the upper grades. Don't get me wrong, some of them are potty trained but it can be a big chore getting them to that point. I also have an Autistic brother who was not fully trained until long after 7 yrs old, and he never got fully trained to poop in the toilet. Don't feel too bad about him sitting in his messy pull-up. Many autistic kids don't sense things the way we do, so things that may seem really gross to us don't bother them, while some things seem terrible to them yet are fine for us. There seems to be sensory issues for a lot of Autistic children.
I do not recommend forcing him to sit on the toilet. That will get him frustrated, and the more frustrated he is the harder it will be to get him back down. If you can find something that doesn't get him upset, try that. In general you want to avoid things that set him off if it can be done reasonably. If they are things he needs to get used to then you expose him in small increments, then gradually increase exposure as he does better in that situation.
With the potty, I would try starting with having him go regularly (every hour or so) and sit him down just for the time he is willing to sit. Don't push the issue, even if he will only sit for half a minute. You don't want to make it into something negative for him. Just keep trying and encourage him to potty, and each time he sits for even a short time congratulate him for sitting on the potty. After a while he should get used to sitting for longer spans of time and hopefully will, at some point, use the potty. When he does use it, even if it's just a bit let him know what a great job he did. Make sure he knows that it is awesome that he went potty in the toilet like a big boy!
It is also probably a really good idea to follow the other recommendations given buy several people to let him see daddy use the potty. The more he sees daddy do this, and understands its fine, the more likely he will be encouraged to try also. I really liked the idea of siting on the potty while reading. I think giving him something like that, that can distract him for a while, may get him to stay on the toilet longer.
What area do you live in? I'm in Palm Bay and one of my friends who has a son with Autism has recently started an Autism support group here. I help out, and attend the meetings. It has been very informative and helpful so far. She has childcare during the meetings so the kids are taken care of, and they all seem to enjoy it. She is also putting together other activities in the community, to get the families out to have fun with the kids. Anyway, if you are in the area, I think it's a great resource. If you or anyone else wants more information, you can post a message to me and I will pass the information along.